Screen frames are commonly used in the printing industry. A printing screen (e.g., a silk screen) is stretched tightly over the frame and is then secured to the frame. The screen includes a desired image or pattern to be printed. In order to obtain the desired image or pattern on the screen, the screen is coated with a liquid emulsion which must be dried before the screen can be used for printing.
In order to dry the emulsion properly, the screen (secured to its own frame) must be supported (with the screen side down) in a horizontal plane.
A properly-coated screen must have an emulsion profile which is "proud", i.e., it is higher than the surface of the screen/mesh which carries it. The thickness of the emulsion over the screen should be a minimum of four microns on the substrate or transfer side of the screen printing frame. This minimum thickness of dried emulsion over the screen/mesh is to create a wall that defines the edges of the image, thereby making a stencil. The stencil will then determine the shape of the ink being transferred. The stencil's thickness over the mesh also acts as a stilt to suspend the mesh above and away from the substrate or material to be printed. It is important that the mesh not be permitted to make contact with the substrate or else the threads of the mesh will influence the shape of the ink deposited. This would be highly undesirable. The mesh is intended to be merely the carrier of the stencil. The thickness of the emulsion on the mesh is achieved through proper coating technique and is secured by proper drying position.
The frame must be dried with the screen or mesh side down to allow the force of gravity to pull and hold the liquid emulsion below the mesh while drying. If the mesh is dried while on the top side of the frame, gravity will pull and force the emulsion in its liquid state to flow down through the mesh to its bottom side inside the frame. This is undesirable because if the emulsion is on the backside of the mesh it will not allow the ink squeegee to make surface-to-surface contact with the mesh during its transfer stroke. If the ink is not cut off at the top of the mesh it will not be free to transfer and stay with the substrate when the screen is lifted off the substrate. The result is unfortunately a misprint.
The minimum thickness of the emulsion on the mesh should be four microns because the average larger particle size of most ink systems is four microns.
Conventional racks which have been used for supporting screen frames include flat ledges in a horizontal plane on opposite sides of an upright frame. However, there are a number of disadvantages associated with the use of such racks. For example, if the width of the screen frame is less than the distance between the ledges on the opposite sides of the rack, then the screen frame cannot be supported by the rack at all. Thus, racks of different sizes are required in order to accommodate screen frames of different widths. Also, if the ledges on the racks are too wide, the emulsion coated on the screen will touch the ledges. This is very undesirable because the screen can become bonded to the ledges when the emulsion dries. Furthermore, even if the screen doesn't become bonded to the ledges, the image area on the screen will be ruined in areas where it is contacted by the ledges. Moreover, where the screen contacts the ledges, water cannot evaporate from the screen.
Conventional racks have flat level ledges about one to two inches wide. This allows only one size frame to be accommodated on a given rack. Surface-to-surface contact between the flat ledges and the screen frame traps moisture, thereby inhibiting evaporation of water or chemicals.
The large amount of surface contact between the screen frame and the ledges also creates significant friction between the screen frame and the ledges when the frame is slid into the rack. This friction wears and weakens the mesh, allowing it to break easily and prematurely. The edges of the ledges can also tear the mesh immediately and render it useless.
There has not heretofore been provided a rack system which can safely and effectively support a variety of screen frames of various widths.